You have the following entry in the tnsnames.ors file of your hq.us.example.com host machine: ORCL = (DESCRIPTION = (ADDRESS_LIST = (ADRESS = (PROTOCOL = TCP)(HOST = hq.us.example.com)(PORT = 1521))…

While NAMES.DEFAULT_DOMAIN affects how unqualified net service names are resolved, ORCL is already a fully defined entry in tnsnames.ora, so this parameter is not the determining factor for this connection.

The net service name ORCL is resolved by exact match lookup in the local tnsnames.ora file and does not require a domain suffix to be appended for successful resolution.

The SERVICE_NAMES initialization parameter controls how the database registers its services with the listener, but the connection ultimately requires the service ORCL.us.example.com to be actively registered - setting the parameter alone does not guarantee registration has occurred.

The tnsnames.ora entry for ORCL maps to SERVICE_NAME = ORCL.us.example.com. For SQL*Plus to successfully connect using this alias, the Oracle listener must have the ORCL.us.example.com service registered - either through dynamic registration by the PMON process or via static listener configuration. Without this service registration, the listener cannot route the connection.